The invention relates to methods and apparatus for measuring the magnitude of indentations made for testing the hardness of the surfaces of solid materials, and more particularly to such methods and apparatus which scan the indentations with a light source and measure the reflections thereon.
It is known in principle how to determine the magnitude of indentations, made by hardness testing equipment, with the help of opto-electronic devices. For instance, according to the German Patent disclosure OS No. 23 31 124, the solid surface containing an indentation is illuminated by a light beam in a specified direction. The portion of the light beam that is incident on the indented area is reflected in directions which differ from the direction of the reflected light coming from the undisturbed surface. This effect is utilized for generating an optical image of the indent and its surrounding region. This image is then quantitatively evaluated by a scanning technique which counts the number of picture elements in the image so generated.
One disadvantage the known scanning methods have is that they must rely on an enlarged image of the indentation. Consequently, the quantitative result of the measurement depends on the magnification factor of the magnifying system which has to be known and maintained constant. Furthermore, working with enlarged images of the indentation involves relatively large scanning fields which may create linearity problems. The lens aberrations must be considered as another source of possible measuring errors since the geometry of the image is not necessarily identical with the geometry of the indent.